FIG. 9 is an exploded perspective view of a conventional inductor device, FIG. 10 is the perspective view of the device, and FIG. 11 shows impedance-frequency characteristics of the device.
The conventional inductor device includes a magnetic section 1 made of magnetic material, a coil pattern formed of a spiral conductive portion 2 in the magnetic section 1, and an external electrode 3 coupled to the coil pattern electrically.
Plural magnetic layers 4 are laminated to form the magnetic section 1. Each magnetic layer 4 is provided with the spiral conductive portion 2 of the coil pattern having an arc shape of less than one turn. Arc-shaped conductive portions 2 on magnetic layers 4 are electrically coupled through a via-hole 5, thus providing the coil pattern of a few turns in the magnetic section 1.
Conductive portion 2 functions as a common-mode choke coil. FIG. 11 shows impedance-frequency characteristics of the choke coil.
In the conventional inductor device, magnetic section 1 includes plural magnetic layers 4 each having arc-shaped conductive portion 2 thereon are laminated to form the coil pattern in the magnetic section. Therefore, the magnetic material of magnetic section 1 is disposed between conductive portions 2 adjacent to each other on magnetic layers 4 adjacent to each other. Magnetic permeability between conductive portions 2 increases since the layers sandwiches magnetic layer 4, thus increases magnetic flux passing through inside of conductive portion 2 (leakage flux). Magnetic flux passing through the coil pattern decreases accordingly, and this decreases an impedance and resulting insufficient attenuation.
Magnetic material having high permeability generally increases the magnetic flux around the coil pattern, and thus, increase the impedance for preventing attenuation from decreasing.
However, the magnetic material having the high permeability decreases attenuation properties at a high frequency band since a peak of the impedance shifts to a lower frequency band. As shown in the impedance-frequency characteristics in FIG. 11, the inductor device, being used especially as a common mode choke coil, have its attenuation properties decrease in a high frequency band since a peak impedance 6 for a common-mode current, i.e., a noise component, shifts to a lower frequency band. In addition, since a peak impedance 7 for a normal-mode current, i.e., an information signal component, shifts to a lower frequency band, the information signal component attenuates in a lower frequency band.
Magnetic layers 4 are pressed against the coil patterns in their laminating process. For this process, a cross-section of the conductive portion must have a stripe shape having its lateral size smaller than its thickness, so that magnetic layer 4 may be placed easily between conductive portions 2 of the coil pattern.
This configuration, however, increases an area of conductive portions 2 placed on magnetic layers 4 facing each other, and generates stray capacitance in the area. The capacitance decreases the attenuation properties in a high frequency band since the peak impedance shifts to a lower frequency band.
As mentioned above, the conventional inductor device has the decreased attenuation properties in a high frequency band, and hardly have a low profile since a lot of magnetic layers 4 are necessarily be stacked to have the coil of only a few turns.